create_loc_conf must return NULL on error

All create_(main|srv|loc|*)_conf methods must return NULL on error, not NGX_CONF_ERROR.

See nginx revision:

changeset:   2912:c7d57b539248
user:        Igor Sysoev <igor@sysoev.ru>
date:        Tue Jun 02 16:09:44 2009 +0000
summary:     return NULL instead of NGX_CONF_ERROR on a create conf failure

And also see ngx_http_core_server at src/http/ngx_http_core_module.c around 2985 and 2996. If value is not NULL, conf is accepted and causes errors later.